A patent document 1 discloses a technology for detecting an operating position of a piston of a fluid pressure cylinder using a magnetostriction type position detecting device. The position detecting device detects a position of a permanent magnet by using a magnetostrictive line composed of a ferromagnetic material and the permanent magnet, generating ultrasonic oscillation to the magnetostrictive line at a position corresponding to the permanent magnet by the mutual action between a magnetic field generated when a current pulse flows to the elastic layer and a magnetic field generated by the permanent magnet, and detecting the ultrasonic oscillation traveling in the magnetostrictive line by a receive coil (detection coil). Then, an operating position in the entire stroke of the piston is detected by attaching the permanent magnet to the piston and the magnetostrictive line to a cylinder tube.
Incidentally, in the technology disclosed in the patent document 1, the position detecting device has a metal probe in which the magnetostrictive line is accommodated, and the probe is fitted into a groove of the cylinder tube composed of aluminum alloy and the like in an electrically insulated state. That is, the probe is formed by inserting the magnetostrictive line into a cylindrical pipe composed of a conductive material such as metal and the like, electrically connecting the extreme end of the magnetostrictive line to the extreme end of the cylindrical pipe, and disposing the detection coil to the base end of the cylindrical pipe. The outer periphery of the probe is covered with an insulation tube, and the probe is fitted into the groove of the cylinder tube in the electrically insulated state through the insulation tube. Then, the cylindrical pipe functions as a feedback conductor of the current pulse supplied to the magnetostrictive line. Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-329409